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Previously on MFMH:
* Our hero added incremental kernel export updates to mountd so
that a single update would finish within his lifetime,
* Hero subsequently discovers that an incremental update still
takes 8 seconds on the Emulab Mothership,
* Hero discovers that remaining time is spent NOT in kernel operations
or even complex calculations of orbital mechanics, but rather in simple
parsing of text from a 1300 line file.
* Hero gives up in despair.
This week on MFMH:
* Our revitalized hero applies gprof-fu to the problem and
discovers that 7.9 seconds of the 8 second /etc/exports parse time
is spent stat()ing /etc/nsswitch.conf 2,392,057 times.
* "WtF!", exclaims our hero, "that is 27.3 stats for every text
character in the file!"
* Problem is traced to the fact that we do "-maproot=root" for each
of the 1344 exports and every parse of that option looks up root
in the password file and determines which of the 1784 groups in
/etc/group root belongs to. Each of those 1344 * 1784 "get group
entry" calls, stats /etc/nsswitch.conf to see if the file has
changed.
* "Bite me!", sez Mike, and he adds a CACHE_ROOT_CREDS option to
mountd so that the lookup is only done on the very first instance
of -maproot=root.
A single incremental update of exports now takes 0.75 seconds.